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On Wikipedia it states:

In cellular metabolism, unsaturated fat molecules contain somewhat less energy (i.e., fewer calories) than an equivalent amount of saturated fat.

Can someone explain why that is true in simple words? (that is, if possible, in a way understandable by someone who is not conversant with biology nor chemistry)

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    $\begingroup$ I think the 'energy' referred to here is Gibbs free energy. One way to think of things, admittedly an over-simplification, is as follows: all of the ATP generated by oxidative phosphorylation is generated by electrons 'held' in C-C and C-H bonds. None comes from water or C-O bonds. Count the total number of C-C plus C-H bonds in the saturated and unsaturated fatty acids. Saturated fatty acids have the potential to 'make' more ATP compared with their unsaturated equivalents, and so far as cellular metabolism is concerned may be thought of as a better 'source' of free energy $\endgroup$ – tomd Oct 10 '16 at 20:09
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The total energy stored in macromolecules is commonly quantified by the amount of $\ce{ATP}$ produced, as $\ce{ATP}$ is used throughout the body to couple reactions and make them thermodynamically favorable.

First, as the oxidation of a double bond in an fully saturated fatty acid requires a reducing agent, you here produce $\ce{FADH2}$ from $\ce{FAD+}$ which is equivalent to a $\ce{1.5 ATP}$ gain.[1]

Also, unsaturated fat has double bonds in cis positions that are unfavorable to beta oxidation.[2] An enzyme coupled reaction is needed for the double bond shift. Therefore, an enzyme coupled reaction that requires energy (in the reductant form $\ce{NADPH}$) is needed to make the molecule a trans fat with a shift in bond position in order to continue oxidizing the molecule completely. $\ce{NADPH}$, usually is defined as $\ce{2.5ATP}$ equivalents[1] and this extra energy expenditure in shifting the double bond is why less energy is ultimately produced. Note, this $\ce{NADPH}$ is only used when unsaturated fats have an even numbered bond position.[2] This happens with polyunsaturated fats when the first bond is shifted.

For example:

18:2 linoleic acid will yield $1.5 + 1.5 + 2.5 = 5.5$ less $\ce{ATP}$ than 18:0 stearic acid.


References:

[1]: http://www.saylor.org/site/wp-content/uploads/2013/04/BIO101A-6.2.3-CellularRespiration.pdf

[2]: https://en.wikipedia.org/wiki/Beta_oxidation

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Yes, it is a fact that saturated fats contain more gross energy due to higher H+ concentration. However, unsaturated fats are more digestible and have higher absorption rate, and therefore the metabolizable energy that unsaturated fats provide is higher than that provided by saturated fats.

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